Methanol production is typically accomplished by direct hydrogenation of carbon monoxide and, to a lesser extent, carbon dioxide according to the following equations: EQU CO+2H.sub.2 .revreaction.CH.sub.3 OH EQU CO.sub.2 +3H.sub.2 .revreaction.CH.sub.3 OH+H.sub.2 O
These reactions are most commonly carried out by contacting the carbon monoxide, carbon dioxide and hydrogen reactants with a fixed bed of catalyst. Unfortunately, in fixed bed systems, conversion yields are generally in the range of only 15 to 30% due to equilibrium limitations. Consequently, large volumes of unconverted reactants must be recycled to the reactor. Applicants and others have investigated alternate systems to increase the single pass conversion and to decrease or eliminate reactant recycle.
U.S. Pat. No. 4,731,387 discloses a method of producing methanol using a reaction zone of a fixed bed of catalyst particles with interstices between the particles. Fine particles capable of adsorbing substantially all of the methanol product are passed in a downward direction through the interstices. The fine particles adsorb the methanol and are ultimately withdrawn from the reaction zone. Methanol is desorbed and recovered from the particles that were withdrawn from the reaction zone. Through adsorbing and removing the methanol product from the reaction zone, a high driving force for the reaction towards methanol production is maintained and conversion yields are increased.
U.S. Pat No. 4,968,722 discloses a similar methanol production process using a fixed bed of catalyst particles with a liquid flowing through the bed to absorb methanol. The patent teaches that it is expected that the liquid film will cause mass transfer resistances at the catalyst surface and especially inside the pores resulting in a loss of catalyst activity. Therefore, the stated preferred embodiment is one where the fixed bed of catalyst is in a reaction zone which is connected to an absorption zone where the methanol is absorbed. The system may consist of a single reaction zone, the effluent of which is passed through a single absorption zone to remove methanol before being recycled to the reaction zone, or a plurality of reaction zones with the effluent of each reaction zone being passed through an absorption zone for methanol removal before continuing to the next successive reaction zone.
U.K. Patent Application 2,255,516A discloses a methanol production process using a plurality of fluidized bed catalytic reactors in series and an inert solvent to absorb methanol from the reaction mixture. Between consecutive fluidized bed reactors a facility is present to effect intimate contact between the inert solvent and the methanol-containing reaction mixture so that the methanol is absorbed by the inert solvent and the methanol-depleted reaction mixture is conducted to the next successive fluidized bed reactor. The methanol is recovered from the inert solvent through flashing and subsequent distillation. The application teaches that suitable inert solvents are water, higher alcohols, higher esters, and higher ethers.
U.K. Patent Application 2,344,329A discloses a methanol production process where synthesis gas is produced and fed to a conventional methanol synthesis reactor where a portion of the synthesis gas is convened to methanol which is collected. A very large purge gas stream from the conventional reactor is then conducted to a cyclically operated vessel containing a mixture of methanol synthesis catalyst and methanol adsorbent. In this vessel, as methanol is formed it is adsorbed, thus removing it from the reactants and promoting more methanol to be formed. When the adsorbent is saturated with methanol, the vessel is taken offline and the methanol desorbed by dcpressurizing or sweeping with methane. The desorbed methanol is combined with the methanol collected from the conventional reactor.
Applicants' invention is significantly different from tile disclosures of the prior art. For example, applicants use a mixture of a methanol synthesis catalyst and a methanol adsorbent in a simulated moving bed mode. The mixture allows the methanol to be adsorbed immediately upon its formation, thereby removing the menthanol from the reactants and permitting the formation of methanol to continue. The simulated moving bed provides a process for not only synthesizing and adsorbing methanol, but also for desorbing the separated methanol using a desorbent, all within the same system. Furthermore, since the methanol is desorbed while still in the presence of the catalyst, specific desorbents may be chosen which aid in suppressing the back reaction of methanol to carbon monoxide, carbon dioxide, and hydrogen. Additionally, two suitable desorbents are reactants themselves, facilitating the increased conversion and eliminating the need for an inert desorbent. The desorbent also may be introduced to the simulated moving bed at a temperature which quenches the back reaction of methanol to carbon monoxide, carbon dioxide, and hydrogen.